a. Field of the Invention
The present invention relates to a process for preparing molded articles of fiber-reinforced thermoplastics (hereinafter abbreviated as FRTP).
b. Description of the Prior Art
Conventionally known FRTP which contains a thermoplastic resin as a matrix material is reinforced with short fibers having a fiber length of no greater than 1 mm, or with continuous strands of mat.
FRTP which contains a thermoplastic resin as a matrix material and is reinforced with short fibers having a fiber length of no greater than 1 mm has good fiber flowability and hence is widely used for injection molding materials. The molded products have good appearance but poor strengths.
FRTP which contains thermoplastic resin as a matrix material and is reinforced with continuous strands of mat has poor fiber flowability and hence is used for stamp molding materials which are primarily used for molding almost flat forms. The molded forms have poor appearance and high strengths.
A new material having combined characteristics of the above two materials has recently been developed. The new material has good fiber flowability and provides both good appearance and high strength to the molded products. In processing the new material, strands or yarns are cut into a length of about 2.5 mm. The resulting chopped strands or chopped yarns are stacked to a desired shape and molded by below described press molding, autoclave molding or stamp molding.
However, FRTP containing the chopped strands or the chopped yarns inevitably generates non-uniform fiber flow or orientation of fiber, which leads to scattering of strengths and warpage or torsion of molded products. Thus, FRTP has many problems with respect to reliability when used for structural materials.
Chopped strand mats or chopped yarns useful for stamp molding have been disclosed. Woven textiles, however, cannot be applied to the reinforcing fiber in the stamp molding process. Woven textiles are different from chopped strands or chopped yarns and have almost no degree of freedom with respect to fiber flowability, and hence uniform filling of the mold is inhibited. When the shape of the molded products, i.e. the configuration of the cavity in the mold, is similar to a plate, stamp molding, press molding or autoclave molding can also be applied. However, in three-dimensional configurations such as curved surfaces, partial overlapping of the woven textiles during the molding process, thereby causing an excess of fiber and a decrease in strength in the overlapped portion.